How to Make It
In a large pot, heat the vegetable oil until shimmering. Add the shrimp shells and cook over moderately high heat, stirring, until pink and fragrant, about 3 minutes. Add the onion, carrot and celery and cook until the vegetables have softened, about 3 minutes. Pour in the chicken stock and bring to a boil. Reduce the heat, cover and simmer for 45 minutes. Remove from the heat and let steep, covered, for 30 minutes longer. Strain the stock into a heatproof bowl through a fine sieve, pressing on the solids to extract as much liquid as possible. Wipe out the pot.
In the same pot, melt the butter over moderately high heat until foaming. Add the garlic and onion and cook, stirring, until translucent, about 2 minutes. Add the tomato paste, thyme, bay leaves, celery salt and 2 teaspoons of pepper and cook, stirring constantly, until thick and slightly darkened, about 3 minutes. Stir in the celery and cook for 2 minutes, until slightly softened.
Add the shrimp stock in 3 batches, stirring well after each addition. Bring to a boil, then reduce the heat to moderate and simmer until slightly reduced, stirring occasionally, about 5 minutes. Add the shrimp, the 2 tablespoons of hot sauce and three-fourths of the scallions and cook just until the shrimp are pink throughout, 3 to 5 minutes. Discard the bay leaves. Stir in the lemon juice and season with salt. Transfer to a bowl, sprinkle with the remaining scallions and serve with steamed rice, passing lemon wedges and hot sauce at the table.